Diesel vehicles may be equipped with an exhaust gas treatment system which may include, for example, a urea based selective catalytic reduction (SCR) system and one or more exhaust gas sensors such as nitrogen oxide (NOx) sensors, at least one of which may be disposed downstream of the SCR system. When the SCR system becomes loaded with urea to a point of saturation, which varies with temperature, the SCR system may begin to slip ammonia (NH3). The ammonia slip from the SCR system may be detected by the NOx sensor as NOx resulting in an inaccurate NOx output which is too high. As such, an efficiency of the SCR system may be determined to be lower than it actually is.
The inventors herein have recognized the above issue and have devised an approach to at least partially address it. Thus, a method for an engine system is disclosed. The method includes, responsive to a threshold increase in NOx indicated by an exhaust gas sensor, reducing an amount of exhaust gas recirculation (EGR). The method further includes, while the amount of exhaust gas recirculation is reduced, allocating output from the sensor to each of NH3 and NOx and in different amounts depending on a change in sensor output. The method further includes, adjusting one or more operating parameters based on the allocation and the change in sensor output.
By temporarily reducing the amount of EGR, NOx emission from the engine may increase. If the threshold increase in NOx is due to NOx passing through an SCR catalyst (e.g., entering an upstream end of the SCR catalyst and exiting a downstream end of the SCR catalyst) disposed upstream of the sensor, the sensor may detect an increase in NOx when the EGR is reduced. As such, the sensor output may be allocated to NOx. On the other hand, if the threshold increase in NOx is due to NH3 slip, the sensor output may not change (or may change less than a threshold amount) when the EGR is reduced due to reduction of NOx by NH3 in the SCR catalyst. As such, the sensor output may be allocated to NH3. When the sensor output is allocated to NOx, one or more operating parameters, such as amount of EGR, may be adjusted to reduce the amount of NOx. When the sensor output is allocated to NH3, one or more operating parameters, such as urea injection amount, may be adjusted to reduce the NH3 slip. In this way, errors in calculation of an efficiency of the SCR catalyst may be reduced, and the efficiency of the system may be improved and NH3 slip may be reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.